I. Field of the Invention
The present application relates to mobile communications systems. More specifically, the present invention relates to a novel and improved method and apparatus for merging predetermined neighbor lists to provide a single list of neighboring base stations to mobile communication units, used to identify possible handoff candidates in a code division multiple access (CDMA) cellular communications system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, (the '307 patent) entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and U.S. Pat. No. 5,103,459, (the '459 patent) entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and incorporated by reference herein.
In the '307 and '459 patents, a multiple access technique is disclosed wherein a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as base stations, or cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
In the exemplary CDMA cellular communication system, each base station transmits a "pilot carrier" signal or, for short, pilot signal. The pilot signal is an unmodulated, direct sequence, spread spectrum signal transmitted at all times by each base station using a common pseudorandom noise (PN) spreading code. The pilot signal allows the mobile stations to obtain initial system synchronization, i.e. timing, in addition to providing a phase reference for coherent demodulation and a reference for signal strength comparisons between base stations for handoff determination.
The pilot signal is transmitted by each base station using the same PN spreading code but with a different PN offset. For example, in the present invention the pilot signal spreading code is of a PN code length of 2.sup.15. In this example there are 511 different offsets from the zero offset, where the offsets are in increments of 64 PN chips. It is this PN offset which allows the pilot signals to be distinguished from one another by the mobile station, resulting in a differentiation between base stations from which they originate. Use of the same pilot signal code allows the mobile station to find system timing synchronization by a single search through all pilot signal code phases. The strongest pilot signals, as determined by a correlation process for each PN offset, are readily identifiable. The identified pilot signal generally corresponds to the pilot signal transmitted by the nearest base station. However, due to topography, this may not always be true. This phenomenon causes problems with present methods of generating the Neighbor List Update Message, described below. Further details of pilot searching techniques are disclosed in U.S. Pat. No. 5,577,022 (the '022 patent) entitled "PILOT SIGNAL SEARCHING TECHNIQUE FOR A CELLULAR COMMUNICATIONS SYSTEM," assigned to the assignee of the present invention and incorporated by reference herein.
Each base station also transmits a sync channel signal which is a modulated, encoded, interleaved, direct sequence, spread spectrum signal used by the mobile stations to acquire additional synchronization, system time, and other overhead control information. Information such as system identification, network identification, a pilot PN sequence offset index, a long code state, current system time along with other time parameters and paging channel data rate are transmitted on the sync channel. It should be noted that the pilot PN sequence offset index identifies an offset value from a zero offset pilot PN sequence. The sync channel signal is despread using the same pilot PN sequence offset as the pilot channel.
Each base station also transmits a paging signal on one or more paging channels. The paging signals are modulated, interleaved, scrambled, direct sequence, spread spectrum signals, which contain control and overhead information. The paging channel is used as such to communicate global and mobile station specific orders, including pages. Overhead messages transmitted on the paging channel include a system parameters message, which contains general system and base station overhead information; an access parameters message, which contains information to be used by the mobile station on an access channel when accessing the system; and a Neighbor List Update Message which identifies to the mobile station the pilot signal PN sequence offsets of neighboring base stations. The sync channel transmitted long code state message is used by the mobile station to descramble the paging channel scrambled signal. Like the sync channel signals, the paging channel signals are spread and despread using the same pilot PN sequence offset as the pilot channel.
Each base station transmits user information to an intended mobile station on a selected one of a plurality of traffic channels. Each mobile station is thus assigned to a unique traffic channel for receiving the mobile station specific information. The traffic channel signals are modulated, interleaved, scrambled, direct sequence, spread spectrum signals transmitted to mobile stations on a respective traffic channel. Information received in the sync channel message is used by the mobile station to descramble the traffic channel scrambled signal.
Further details on the modulation scheme for the various channels of the base station are described in the '459 patent.
In present CDMA systems, the mobile station uses multiple receivers, or diversity reception, to combat the problem of multipath fading. Using a diversity receiver, a mobile station simultaneously demodulates a plurality of signals transmitted by a given base station but which have traveled along different propagation paths. The plurality of demodulated estimates are combined to provide an improved estimate of the data. An example of a diversity receiver is disclosed in U.S. Pat. No. 5,109,390 (the '390 patent) entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein. Diversity reception in the mobile station also allows it to communicate with more than one base station at a time, such as when the mobile station is transitioning between cells. This is an important concept for soft handoffs, discussed below.
Handoff is a general term used to describe the process whereby communications between a mobile station and a base station controller, also known as a Mobile Telephone Switching Office (MTSO), is continued as the mobile station travels through more than one coverage area or cell. In the conventional cellular telephone system, the handoff scheme implemented is intended to allow a call to continue when a mobile telephone crosses the boundary between two cells. The handoff from one cell to another is initiated when the receiver in the cell base station handling the call notices that the received signal strength from the mobile telephone falls below a predetermined threshold value. A low signal strength indication implies that the mobile telephone is near the cell border. When the signal level falls below the predetermined threshold value, the base station asks the system controller to determine whether a neighboring base station receives the mobile telephone signal with greater signal strength than the current base station.
The system controller in response to the current base station inquiry sends messages to the neighboring base stations with a handoff request. The base stations neighboring the current base station employ special scanning receivers which look for the signal from the mobile telephone on the specified channel. Should one of the neighboring base stations report an adequate signal level to the system controller, then a handoff will be attempted.
Handoff is then initiated when an idle channel from the channel set used in the new base station is selected. A control message is sent to the mobile telephone commanding it to switch from the current channel to the new channel. At the same time, the system controller switches the call from the first base station to the second base station. The above described handoff method is known as a "hard" handoff because there is no overlap in call continuation as the call is transferred from the first base station to the second base station. The switching function of the hard handoff can be considered a "break-before-make" connection.
In contrast is the soft handoff method used in modern CDMA system. Such methods are disclosed in U.S. Pat. No. 5,267,261 (the '261 patent) entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM" and U.S. Pat. No. 5,101,501 (the '501 patent) entitled "METHOD AND SYSTEM OF PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM", both of which are assigned to the assignee of the present invention and are incorporated by reference herein.
In a mobile assisted soft handoff, the mobile station constantly searches for strong pilot signals. To streamline the process of searching for pilots, four distinct sets of pilot offsets are defined: the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set identifies the base station(s) or sector(s) through which the mobile station is communicating. The Candidate Set identifies the base station(s) or sector(s) for which the pilots have been received at the mobile station with sufficient signal strength to make them members of the Active Set, but have not been placed in the Active Set by the base station(s). The Neighbor Set identifies the base station(s) or sector(s) which are likely candidates for the establishment of communication with the mobile station. The Remaining Set identifies the base station(s) or sector(s) having all other possible pilot offsets in the current system, excluding those pilot offsets currently in the Active, the Candidate and Neighbor sets.
In system communications between the mobile station and the MTSO, the mobile station is provided a Neighbor List Update Message comprising of PN offsets corresponding to base stations of neighboring cells. In addition, the mobile is provided with a message which identifies at least one pilot corresponding to a base station through which the mobile station is to communicate through. These lists are stored at the mobile station as a Neighbor Set and an Active Set of pilots respectively, and are updated as conditions change.
A search is conducted for determining the signal strength for each pilot identified in the Active, Candidate, Neighbor, and Remaining Set as follows. In one possible search technique, the signal strengths of all pilots in the Active and Candidate Sets are measured first, then the signal strength of the first pilot in the Neighbor Set is evaluated. Then the Active and Candidate Set pilots are re-evaluated, followed by the second pilot in the Neighbor Set. This continues until the last Neighbor pilot is evaluated, then the first pilot of the Remaining Set is evaluated. This cycle continues by a re-evaluation of the Active and Candidate pilots and so on. Further details of the searching process are disclosed in the aforementioned '022 patent.
When communications are established with the mobile station through a base station, the mobile station continues to monitor the signal strength of identifiable pilot signals transmitted from the various neighboring base stations. When a pilot signal exceeds a predetermined threshold level in signal strength, denoted as T.sub.-- ADD, the pilot is added to the Candidate Set at the mobile station and a message is sent to the MTSO identifying all Active and Candidate pilots, their respective signal strengths and phase delays. This message is called the Pilot Strength Measurement Message. The phase delay information is a measurement used by the MTSO to determine each pilot's Round Trip Delay time with respect to the mobile station. The MTSO, upon receipt of the Pilot Strength Measurement Message, places the pilot that has exceeded T.sub.-- ADD into the mobile station Active Set and sends a message to the mobile station containing all the pilots to be included in the Active Set through which the mobile station is to communicate. The MTSO also communicates information to the base station corresponding to the new pilot in the Active Set and instructs this base station to establish communications with the mobile station. The mobile station communications are thus routed through all base stations identified by pilots in the mobile station Active Set.
When the mobile station is communicating through multiple base stations, the pilot signal strength of these base stations along with other base stations is monitored by the mobile station as described above. Should a pilot signal corresponding to a pilot of the Active Set drop below a predetermined threshold, denoted as T.sub.-- DROP, for a predetermined period of time, denoted as T.sub.-- TDROP, the mobile station generates and transmits the Pilot Strength Measurement Message to report the event. The MTSO receives this message via at least one of the base stations through which the mobile station is communicating. The MTSO may then decide to terminate communications through the base station whose pilot signal strength as measured at the mobile station is below the threshold level.
The MTSO upon deciding to terminate communications through a base station generates a new message identifying the pilots of the Active Set to which the mobile station is to communicate through. In this message which identifies pilots of the Active Set, the pilot of the base station to which communications with the mobile station are to be terminated is not identified. The MTSO also communicates information to the base station not identified in the Active Set to terminate communications with the mobile station. The mobile station, upon receiving the message identifying pilots of the Active Set, discontinues processing signals from the base station whose pilot is no longer in the Active Set. The mobile station communications are thus routed only through base stations identified by pilots in the mobile station's Active Set. In the case where there were previously more than one pilot identified in the Active Set and now only one, the mobile station communicates only to the one base station corresponding to the pilot identified in the mobile station Active Set.
Since the mobile station is communicating with the MTSO via at least one base station at all times throughout the handoff, there is no interruption in communications between them. A soft handoff in communications provides significant benefits in its inherent "make before break" communication over conventional "break before make" techniques employed in other cellular communication systems.
Soft handoffs are far less likely to drop calls than are hard handoffs because a phenomenon referred to as "ping-ponging" is greatly reduced. "Ping-ponging" occurs when the mobile telephone is near the border between two cells. In this situation the signal level tends to fluctuate at both base stations. This signal level fluctuation results in a situation in which repeated requests are made to hand the call back and forth between the two base stations. Such additional unnecessary handoff requests increase the possibility of the mobile station incorrectly hearing the channel switch command or failing to hear the command at all. Furthermore, this situation raises the possibility that the call will be discontinued if it is inadvertently transferred to a cell in which all channels are currently in use and thus unavailable for accepting the handoff.
The soft handoff technique described above can also be used to provide continued communication as a mobile station traverses between different sectors of a single base station. Such a method is disclosed in U.S. patent application Ser. No. 08/405,611, filed Mar. 13, 1995 entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION," assigned to the assignee of the present invention and incorporated by reference herein.
Current methods of generating the Neighbor List Update Message, while reasonable for small systems, are often problematic when used in large metropolitan areas, because they generate a very large Neighbor Set. For example, the Neighbor List Update Message in some systems is generated from the union of neighbor lists for each pilot signal in the mobile station's Active Set. The Neighbor List Update Message is generated the same way whether the mobile station is in soft handoff or not. The union of neighbors from the Active pilots can yield a very large Neighbor Set which imposes a search burden on the mobile station. That is, the mobile station can only search a limited number of pilot signals within a given time frame. The number of pilots in the Neighbor List Update Message is limited to a predetermined number, such as twenty.
Another problem with present methods of generating the Neighbor List Update Message is false indications of pilot strength due to topography. For example, a mobile station may detect a strong pilot signal from a base station which is located many miles away only because that base station is located on top of a mountain. Or a mobile station may detect a weak pilot signal from a nearby base station only because the mobile station happens to be in a valley at the time the nearby base station pilot energy is measured. In some systems, the Neighbor List Update Message is generated by simply selecting the first pilot signal identified in the Pilot Signal Measurement Message and placing all of its corresponding neighbors into the Neighbor List Update Message first. Then the next pilot's neighbors are placed in the List and so on. Using this method, far away base stations may be included in the Neighbor List Update Message though unsuitable for handoff, while nearby base stations may be excluded, though more suitable for handoff.
What is needed is a better method to generate the Neighbor List Update Message in order to find the most likely candidates for handoff. The method should prioritize the list of neighboring base stations and solve the problem of false pilot signal strengths due to topography.